The present invention relates to a press-working method in which a blank is cut out from a sheet material and the cut-out blank is press-worked.
An agricultural tractor such as that disclosed in Japanese Patent No. 2003-278176 (FIGS. 1, 3, 8, 11) has a front loader, which is an example of a excavating equipment, connected to a front part of a machine body, wherein the front loader comprises right and left booms, a bucket, and the like. The booms comprise a front structural member, a rear structural member, and an intermediate structural member. The front and intermediate structural members and the rear and intermediate structural members are respectively connected by welding.
The front and rear structural members are bent to have a U-shaped cross section, and a flat-plate lower wall is connected to a side wall (wall) by welding. In this case, as described in FIGS. 15-16, the front and rear structural members of the booms are blanks (31) having a predetermined shape that have been cut out from a sheet material and then bent by a pressing device (32) so as to have a U-shaped cross section to whose rear a lower wall is connected by welding.
The upper walls (bottom portions) of the front and rear structural members are configured to have a flat plane surface. Over the past several years, proposals have been made for the upper walls (bottom portions) of the front and rear structural members to be endowed with an arched surface in order to improve the strength and design of the booms.
In this case, when the blank is bent into a U-shape by the pressing device so that the upper walls of the front and rear structural elements have an arched surface while the blank is bent, the side walls (31b) of the front and rear structural members are compressed along the surfaces, and the side walls (31b) of the front and rear structural elements are expected to form a rippled state (be in a wrinkled state, or a state in which there are deformations in the board thickness direction).
A rippled state (a wrinkled state, or a state in which there are deformations in the board thickness direction) of the side walls (the walls of the blank) of the front and rear structural members must, therefore, be prevented by having changes in the board thickness of the side walls (the walls of the blank) of the front and rear structural members controlled by inside and outside molds, as disclosed in Japanese Patent No. 6-142776 and Japanese Patent No. 2002-102934.
In the abovementioned manner, when the blank (31) is bent to have a U-shaped cross section and the bottom of the blank is bent into an arched surface (31a), the edges of the walls (31b) of the blank will extend along the inside and outside molds if changes in the thickness of the walls (31b) of the blank are controlled by the inside and outside molds. Therefore, the expectation is that the edges of the walls (31b) of the blank will deviate from the predetermined shape (will become larger than the predetermined shape).
Therefore, when the blank is in such a state, the edges of the walls of the blank must be machined (e.g., cut with a laser) to yield the predetermined shape. In this case, machining is difficult if the edges of the walls of the blank are not flat but are in a bent state, and inevitably also entails increases in both the number of production steps and the production costs.